In an ion source which generates a ribbon-like ion beam, usually, plural cathodes are used in order to generate a plasma of high uniformity.
The lifetimes of the cathodes depend on sputter due to collisions of ions in a plasma, thermal evaporation, consumption due to a reaction with a source gas for generating a plasma, and the like.
Among cathodes, in an indirectly-heated cathode in which a cathode member is heated by a filament, usually, the cathode member can be made thicker than a filament cathode (in other words, a directly-heated cathode, the same shall apply hereinafter). Therefore, an indirectly-heated cathode has an advantage that the lifetime can be prolonged. An example of an ion source in which two such indirectly-heated cathodes are used is disclosed in Patent Reference 1.
By contrast, a filament cathode is simple in structure, and hence has an advantage that many filament cathodes can be disposed in a relatively easy manner. An example of an ion source in which a plurality of such filament cathodes are used is disclosed in Patent Reference 2.    Patent Reference 1: JP-A-11-25872 (Paragraph 0015, FIG. 2)    Patent Reference 2: JP-A-2000-315473 (Paragraph 0003, FIG. 8)